1. Field of the Invention
This invention relates to recording media and more particularly, to high density recording and reproducing media comprising a specific type of vertical magnetization film.
2. Description of the Prior Art
Ordinary magnetic recording media, such as magnetic tapes, magnetic sheets, magnetic discs, magnetic cards and the like, employed in audio or video recording and reproducing systems and as data files for computers are made of substrates and magnetic layers formed on the substrate. These magnetic recording media are magnetized in directions parallel to the medium surface. However, the magnetic recording in the horizontal directions involves the increasing action of self-demagnetization of the magnetic recording medium itself when the medium is used for high density recording, resulting in considerable lowering of the reproduction output level.
In order to solve the self-demagnetization problem involved in the high density recording, attention has been drawn to recording media comprising vertical magnetization films in which the self-demagnetization action becomes smaller in high density recording operations. The term "vertical magnetization film" used herein is intended to mean a magnetic film which has an axis of easy magnetization perpendicular to the film surface and whose magnetic moment is perpendicular to the film surface, i.e. the constant of magnetic anisotropy has a positive value. When an intrinsic vertical anisotropy constant is taken as K.perp. and a diamagnetic field is taken as 2.pi.Ms.sup.2 in which Ms is a saturation magnetization, the magnetic anisotropy constant Ku can be expressed as follows: Ku=K.perp.-2.pi.Ms.sup.2. In order that the magnetic layer is a vertical magnetization film, the following inequality must be satisfied: Ku&gt;0 or K.perp.&gt;2.pi.Ms.sup.2. A higher intrinsic anisotropy constant of film than the diamagnetic field will produce more preferable results as magnetic recording media.
For instance, a thin film of cobalt itself containing no impurities has a saturation magnetization of about 1400 emu/cc and its magnetic anisotropy constant is negative in value. Thus, a vertical magnetization film cannot be obtained from cobalt itself. Accordingly, attempts have been made to incorporate various impurities in cobalt, thereby rendering the saturation magnetization small. Several vertical magnetization films of Co alloys have now been proposed including, for example, Co--Cr alloy films obtained by sputtering and vacuum deposition techniques, Co--Ru alloy films by sputtering, and C0--Ni--Mn--P alloy films by the electroless plating technique. In these cobalt alloys, the C axis of the hexagonal closest structure of cobalt is arranged in vertical directions with respect to the film surface, by which the vertical anisotropic energy is ensured by the crystalline anisotropic energy. The saturation magnetization is reduced by the action of impurities contained in the alloys. As a result, the requirement of K.perp.&gt;2.pi.Ms.sup.2 is satisfied. However, a magnetic film composed of, for example, 75 wt% of Co, 20 wt% of Ni, 3 wt% of P and 2 wt% of Mn is positive with respect to the magnetic anisotropy constant but its value is close to zero. Hence, this film does not exhibit magnetic characteristics suitable for use in vertical magnetic recording systems and are thus unsatisfactory.